There has been conventionally a silencer using active noise control, as a technique for reducing noise in an object space where sound made by a noise source is propagated (a noise propagation path). The active noise control is a technique to actively reduce the noise by emitting a canceling sound with the reverse phase and the same amplitude relative to the noise.
The active noise control is to execute a silencing program that realizes a function of an adaptive filter in order to follow up a change in the noise made by the noise source or a change in the noise propagation characteristics. However, in case there is external sound (disturbance sound) other than the noise, which is the sound to be silenced, made by the noise source, the disturbance sound may cause a filter coefficient of the adaptive filter to diverge. As a result of divergence of the filter coefficient, an effect of canceling the noise may be reduced, and further, the canceling sound itself emitted by the silencer may become noise.
To solve this issue, there has been proposed a silencer that, when abnormity such as the disturbance sound is detected, resets parameters such as a transfer function and a convergence coefficient which are to be used for the active noise control, and stops outputting of the canceling sound when the number of abnormity detection times exceeds a prescribed value (refer to Document 1: JP 1998-187201 A).
Document 1 discloses the technique, when the disturbance sound is detected, to continue the processing of updating the filter coefficient while resetting the parameters such as the transfer function and the convergence coefficient to be used for the active noise control. As a result, the filter coefficient may diverge, depending on the disturbance sound, even when the parameters such as the transfer function and the convergence coefficient are reset while the update processing of the filter coefficient is continued. In addition, even though the filter coefficient does not diverge, the filter coefficient may be updated to a value adaptive to the disturbance sound that is not the sound to be silenced, and accordingly, it may cause to reduce the performance of silencing the noise, which is originally the sound to be silenced, for some time after the mixing of the disturbance sound has been lost.